The evolution of organogenesis in mammals (English version)

This animation belongs to the article: “Sánchez-Villagra MR & Werneburg I (2017). Mammalian organogenesis in deep time: tools for teaching and outreach. Evolution: Education and Outreach”. It discusses the research from “Werneburg I, Laurin M, Koyabu D, Sánchez-Villagra MR (2016). Evolution of organogenesis and the origin of altriciality in mammals. Evolution & Development 18(4): 229-244”.

The first part shows the simplified scheme of the phylogenetic framework used here. Images of embryos and fetuses are taken from different sources. Drawings of ancestral placental embryos are based on the reconstruction of the ancestral sequence the embryological characters studied herein.
Part 2 presents a life reconstruction of ten eggs (as reconstructed herein) and one hatchling with its mother of the early amniote Dimetrodon (Synapsida). A focus is laid on the reconstructed closed eyelids at hatching.

Below the text spoken during the animation (speaker: Laura A. B. Wilson).

The evolution of organogenesis in mammals
Extant mammals show a great diversity in body form: humans and dolphins, dogs and bats, elephants and mice.
At birth they already resemble the anatomy of their adults.
The characteristic shape at birth is established in prenatal life, long before birth. Although a human newborn is very different from an adult, a human can be distinguished from its closest evolutionary relatives already at birth.
Using an evolutionary framework illustrating the tree of mammalian life it is possible to compare the development of the organs and general anatomy of a variety of mammalian species.
Here we see embryos of deer, human and rabbit as examples of placental mammals.
We documented the order of appearance of discrete embryological features belonging to the limbs – arms and legs -, heart, eye, ear, and snout development, as well as somites and branchial arches.
A change in the timing of those characters - earlier or later appearance - results in larger or smaller organs.
A specific sequence of events characterizes species.
As development progresses, the differences among deer, human, and rabbit appear.
The comparison can be extended to many more mammalian species…
… and with all the information we can reconstruct the organogenesis of the last common ancestor of all placental mammals. We answer the question: how did the first placentals, which probably lived around the time of non-avian dinosaurs, develop in the uterus?
It was likely born after four months and weaning took place after several weeks.
We can then make comparisons with other mammalian and with reptilian species to better understand what makes placentals unique.
For marsupials, for monotremes (which include platypus and echidna), and reptiles we reconstructed the ancestral organogenesis.
We found that several fully terrestrial land vertebrates open their eye lids long after birth or hatching, which shows that the young are blind for a long time and need help to survive.
Based on those findings, we reconstructed the life history of the last common ancestor of all fully land adapted vertebrates, the amniotes.
We hypothesized based on comparisons among species that that animal laid 10 eggs. We found that the hatchling had closed eyes, like most mammals at birth, and the eyelids opened after 18* days.
In this long period, the young was blind and the mother must have fed and protected the young.

corrected compared to the audio version